President Bakiyev said the new system would give the people more power, but opposition parties say the changes will hand more power to Mr Bakiyev.

The election is expected to take place later this year, probably in December.

Reuters reported some election observers as saying there had been incidents of ballot-stuffing during Sunday's referendum, and the official turnout of 80% of the country's 2.7m voters was inflated.

But the Central Election Commission denied any major irregularities.

Political stand-off

President Bakiyev swept to power in 2005, ousting long-term leader Askar Akayev and winning a landslide victory in presidential elections.

But since then, the Central Asian country has been hampered by a political stand-off between the president and parliamentary deputies elected during the Akayev era.

The new constitution includes a proportional representation system of voting, where candidates are selected from central party lists rather than locally elected.

The president has formed his own party, Ak Zhol, and analysts say the constitutional changes will help him gain a foothold in the chamber.

Other changes to the constitution include giving parliament more power in forming the government, and limiting the president's ability to dismiss parliament. It will also increase the number of deputies.

While President Bakiyev says the changes will help to clarify the separation of power, his critics say they are a move towards authoritarianism.

The former Soviet republic is strategically crucial for the US, whose only remaining military base in Central Asia lies just outside the capital Bishkek.